The objective of the current study is to investigate the efficacy, safety and pharmacokinetics of five doses of BI 10773 compared to placebo given for 12 weeks as add-on therapy to on going metformin therapy in patients with T2DM with insufficient glycemic control. In addition, there will be an open-label treatment arm with sitagliptin (JanuviaTM) as add-on therapy to metformin.